The CDMA technique is used in third generation systems in particular, such as the Universal Mobile Telecommunication System (UMTS) in particular, offering services at bit rates higher than those offered by so-called second generation systems, such as the Global System for Mobile communication (GSM) in particular.
As a general rule these systems have a cellular architecture, and intercellular transfer (handover) techniques are provided for transferring calls from cell to cell as required. Another conventional technique is the Mobile Assisted HandOver (MAHO) technique, in which a mobile station performs radio measurements on broadcast channels that are broadcast in cells adjoining its serving cell and reports the results of those radio measurements to the network, with a view to facilitating a handover decision by the network. A list of the adjoining cells on which the measurements are to be effected is generally indicated to the mobile station by the network.
Also, one technique routinely used in CDMA systems is macrodiversity transmission, whereby a mobile station is connected simultaneously to a plurality of base stations, i.e. is serviced simultaneously by a plurality of cells known as serving cells (or active cells). By using appropriate techniques for processing and combining the signals received, this improves receive performance in particular. It also minimizes the risks of call loss at the time of handovers. For this reason this technique is also known as “soft handover”, as compared to the “hard handover” technique whereby a mobile station is connected at any given time to only one base station.
As general rule, and as outlined in FIG. 1, a mobile radio system includes a radio access network (called the UMTS Terrestrial Radio Network (UTRAN) in the UMTS) in turn including a set of base stations (in the UMTS a base station is referred to as a “Node B”) and a set of base station controllers (referred to in the UMTS as “radio network controllers” (RNCs)). The UTRAN communicates, on the one hand, with mobile stations (a mobile station is also referred to as a “user equipment” (UE)) and, on the other hand, with a core network (CN).
In the case of macrodiversity transmission, the various “Node B” stations to which a UE is connected may or may not be controlled by the same RNC. If they are controlled by different RNC, one of those RNC, called the serving RNC (SRNC), has a control role for the call concerned, including functions of adding or removing serving cells, drawing up up-to-date lists of serving cells and adjoining cells, communicating such lists to the UE, centralized power control, etc. The “Node B” stations connected to the UE and not controlled by the SRNC communicate with the SRNC via the RNC that control them, which are referred to as drift RNC (DRNC). By way of example, FIG. 1 shows the situation of a UE connected to two “Node B” stations controlled by two different RNC, one of which has an SRNC role and the other a DRNC role.
Thus, as shown in FIG. 1, the RNC are connected:                to the “Node B” stations via an interface called the “lub” interface,        to each other via an interface called the “lur” interface, and        to the core network via an interface called the “lu” interface.        
In the case of macrodiversity transmission, the list of adjoining cells allows for the fact that each serving cell itself has adjoining cells. Thus to draw up the list of adjoining cells the SRNC needs to know the cells adjoining cells that it does not itself control, but which are controlled by the DRNC. The information that the SRNC needs on these adjoining cells, referred to hereinafter as “adjoining cell information”, includes parameters such as: adjoining cell identification, identification of the RNC controlling it, transmit power level of the broadcast channel in that cell, etc. The identification of an adjoining cell is used to identify that cell for communication by the SRNC to the UE of the list of adjoining cells on which measurements are to be effected and then for communication of the measurement results by the UE to the SRNC. If necessary, the identification of the RNC that is controlling an adjoining cell enables the SRNC to connect that RNC, on the one hand, to set up a connection in that cell and, on the other hand, to obtain from that RNC the adjoining cells of that cell, in order to enable the process to continue. For the UMTS, for example, the adjoining cell information is specified in the document 3G TS 25.423 Version 3.3.0 Release 99, published by the 3GPP (“3rd Generation Partnership Project”).
Two techniques can then be used. Either all the adjoining cell information necessary to respond to all possible handover schemes is provided directly in the SRNC (and therefore in each RNC, since each RNC can become the SRNC for a given connection), although this has the essential disadvantage of being relatively costly in terms of memory volume and updating adjoining cell information in the case of network configuration changes. Or the DRNCs indicate to the SRNC the adjoining cell information in respect of the adjoining cells of the serving cells that they control, when the SRNC needs it, i.e. in response to a corresponding request made by the SRNC. This avoids the previous drawbacks.
This latter technique is described, for example, in the document WO 00/11878 and is also specified, for the UMTS, in the document 3G TS 25.423 Version 3.3.0 Release 1999 already cited.
However, the above documents are limited to homogeneous systems, i.e. systems in which all the cells use the same radio access technique, here the CDMA technique. The above documents do not cover heterogeneous systems, i.e. systems in which some cells use radio access techniques other than the CDMA technique, in particular the TDMA (Time Division Multiple Access) technique used in the GSM. In other words, the above documents do not cover the case of intersystem handover, in particular from the UMTS to the GSM. For example, in the document 3G TS 25.423, only parameters characteristic of UMTS cells are included in the adjoining cell information contained in “radio link set up response”, “radio link setup failure” or “radio link addition response”, “radio link addition failure” signaling messages transmitted in the DRNC to SRNC direction in response to “radio link setup request” or “radio link addition request” signaling messages transmitted in the SRNC to DRNC direction.
U.S. Pat. No. 5,594,718 describes intersystem handover from a first system using the CDMA technique to a second system using a technique other than the CDMA technique. The base stations of the second system, situated at the boundary with the CDMA system, are adapted to generate a pilot signal as if they were base stations of the CDMA system. The list of adjoining base stations communicated by the CDMA system to a mobile station in this region at the boundary between the two systems then includes such base stations of the second system, and if the mobile station reports the corresponding measurement results to the network, the CDMA network recognizes that these are base stations of the second system, and if appropriate initiates a handover to the second system. A particular drawback of this kind of solution is that it necessitates adaptation of the base stations of the second system and is therefore difficult and costly.